In most electrical circuits, a power switch is connected in series with a load. The function of the power switch is either to interrupt or sustain the current flow from an AC power source to the load. This power switch can be controlled by a control circuit whose function is to either energize or de-energize the load according to a specific function. For example, where the load is one or a plurality of electric lights, the control circuit may be provided for turning on the lights when a person enters a room. In this application, a PIR (passive infrared) motion detector can be used as part of the control circuit. Upon detecting such presence, the detector activates a control circuit which turns the lights on.
While the lights are on, it is also known in the art to use phase control of the solid state power switch to apply the AC supply signal to the load for a controlled fraction of each AC power cycle. For example, U.S. Pat. No. 4,478,468 (Schoen et al) discloses a line-gated switching power supply connected to a control circuit which, in turn, is connected to a solid state switch. The switch is in the form of a triac and is connected in series with a lamp. The control circuit controls the time of firing of the triac to achieve different intensities of illumination from the lamp (i.e. a longer conduction time of the triac during each AC half cycle results in greater intensity of illumination). According to the power supply of Schoen et al, a reservoir capacitor is charged during the portion of the AC cycle during which the triac is non-conducting. While the prior art system of Schoen et al is useful for providing a power supply which operates whether the AC source is continuous or intermittent (i.e. only a portion of each AC cycle being applied to the load), the specification is silent as to how the control circuit determines when zero crossings of the AC power signal have occurred. This timing is critical for proper operation of the triac. Precise detection of zero crossings of the AC power signal can be particularly difficult in the face of AC mains frequency variations. Furthermore, no means are disclosed or suggested for detecting a current overload condition within the main circuit comprising the lamp and triac, and for preventing application of further power to the load in such a condition.